In prior art regarding electric lift trucks, at least two controllers are used, one controller for the electric drive motor which may be a series wound motor or a separately excited motor, for example, and another controller for the hydraulic pump unit. The two controllers require double cabling and a duplication of processors and other components.
In prior art electric lift trucks, it is also common practice to run the hydraulic pump at or near full capacity at all times. Oil is pumped into a hydraulic circuit or a pressure tank and a pressure relief valve bleeds off excess fluid pressure into a reservoir where it is picked up by the pump and re-pressurized in a continuous cycle. As pressurized fluid is required for lift functions, it is delivered by the pump to the working devices and then bled back into the reservoir. While this arrangement works well, it is very energy inefficient.
Another feature of prior art electric lift trucks is that hydraulically-assisted power steering in such trucks is generally provided with a separate power steering pump motor which uses a common reservoir with the lift motor but is otherwise a separate hydraulic circuit.
In most prior art lift trucks, lift functions are controlled by a bank of hydraulic valves which are manually operated to provide such lift functions as fork tilt, horizontal reach of the forks toward or away from the front of the truck, left or right side-shift of the fork rack, and raising or lowering of the forks. Thus at least four hydraulic valves may be provided for lift fork control. Each hydraulic valve is generally actuated by a manually operated lever.
In order to improve the control system of an electric lift truck, efforts have been made in respect of many of the features discussed above. U.S. Pat. No. 5,481,875, entitled APPARATUS FOR CHANGING AND CONTROLLING VOLUME OF HYDRAULIC OIL IN HYDRAULIC EXCAVATOR, which issued to Takamura et al. on Jan. 9, 1996 is an example. This patent discloses a hydraulic oil volume change-over control apparatus for a hydraulic excavator which subjects a hydraulic pump to load sensing control so as to provide an optimum volume of hydraulic oil while an engine for driving the hydraulic pump is operated at a rotational speed at which the fuel consumption of the engine is minimal, by setting a low power mode during breaker work or the like which is performed with a smaller volume of hydraulic oil than that needed during normal excavating work. The control apparatus comprises a variable displacement hydraulic pump, an engine for driving the hydraulic pump, an actuator driven by the hydraulic pump, an actuator control valve disposed in pipe lines between the hydraulic pump and the actuator, a load sensing control device for the hydraulic pump, and a controller for computing a control signal for operating the engine at a minimum fuel consumption rate under a predetermined power designated by the working mode changeover device, so as to deliver a control signal to the load sensing control device and a governor drive device for the engine. The controller can receive signals from a volume sensor for the hydraulic pump, an engine rotational speed sensor for the engine, a hydraulic pressure sensor for the actuator, and the load sensing control device.
Another example is U.S. Pat. No. 4,449,365 entitled LIFT, TILT AND STEERING CONTROL FOR A LIFT TRUCK, which issued to Hancock on May 22, 1984. This patent discloses a lift truck hydraulic control system designed to conserve energy including a pair of separately controlled pumps (21, 22). One pump (21) supplies pressure fluid to a valve (12) for a steering cylinder (11) by way of a high priority port (34) of a priority valve (32) with a low priority flow to parallel connected lift and tilt valves (19, 18) which control operation of the lift cylinder (15) and tilt cylinders (16, 17), respectively. The capacity of pump (21) is sufficient to provide proper, effective operation of the steering and tilt functions but is not adequate to provide hydraulic fluid flow for high- speed extension of the lift cylinder (15). The other pump (22) is operated to supply additional pressure fluid flow for high-speed lift only when the lift valve (19) is shifted to a raise position. In one embodiment, low speed lift is obtained by using the output of the first pump (21) and a high-speed lift is obtained by selectively adding the output of the second pump 22. This is achieved by operating the second pump (22) only when the lift valve (19) is placed in an extreme raise position. In another embodiment, both pumps (21, 22) are operated "on demand", thereby further conserving energy.
Also directed to a control system for power steering, U.S. Pat. No. 3,991,846, which issued on Nov. 16, 1976 to Chichester et al. is entitled POWER STEERING SYSTEM and discloses a power steering system for electric vehicles. The system uses an electric drive motor for the supply pump of a power steering system controlled by a motor control which is in turn controlled by the steering demand of the operator. The motor and pump are operated only during steering operations and only at the power level required for any steering demand.
An example of improvements in vehicle control is taught in U.S. Pat. No. 5,002,454, entitled INTUITIVE JOYSTICK CONTROL FOR A WORK IMPLEMENT, which issued to Hadank et al. on Mar. 26, 1991. This patent discloses a joystick control system for a work implement of earthmoving or material handling vehicles which includes two multi-axis joysticks that provide the operator with an intuitive control interface to the vehicle in an effort to improve the manipulation of the implement with conventional control handles and pedals that do not intuitively correspond to the movement of the implement. The control system also provides a coordinated control system for spatial placement of the end effector of the work implement.
The examples above illustrate the scope of effort that has been invested in this field. However, the prior art does not teach an integrated controller for both traction and hydraulic function control of an electric lift truck. Furthermore, the prior art does not teach using a single joystick as the only interface to control the propulsion of the truck as well as all hydraulic functions of the lift mechanism. Consequently, several significant problems are left un-addressed by the teachings of the prior art. First, modern lift trucks, especially electrically powered lift trucks, are designed to be compact and space for drive and control components is minimal. There therefore exists a need for a compact controller that can control both drive and hydraulic functions so that space required for electrical cabling and controller footprints are reduced. Secondly, energy conservation has become a significant issue. There therefore exists a need for a new hydraulic circuit and pump control that is capable of supplying adequate pressurized fluid for all hydraulic functions, including power steering, with a single pump. Third, the prior art lift trucks with power assisted steering designed to conserve energy shut down the hydraulic pump(s) when there is no demand for pressurized fluid. Consequently, the pump motors are subjected to frequent starts under load. This causes wear on the motor and shortens service life. There therefore exists a need for an efficient hydraulic pump control that minimizes wear and extends pump service life.